In recent years, a liquid metal ion source has attracted attention, because it can emit an ion beam having a high brightness and a fine diameter of the order of submicrons, which provide a possibility that lithography, doping (implantation), etching, etc. involved in semiconductor processes can be conducted without the use of any mask (i.e., by the maskless method) which has conventionally been required or without resort to any chemical means.
The liquid metal ion source operates according to the following principle. First, a source material (liquid metal) which has been melted by means of resistance heating, electron bombardment, laser radiation or the like is fed to an emitter made of a high-melting material such as tungsten (W), molybdenum (Mo), tantalum (Ta) or silicon carbide (SiC) and having a sharply pointed tip. Application of a negative high voltage to an extraction electrode brings about concentration of an electric field at the tip of the emitter. When a high voltage is further applied and reaches a certain threshold value, the liquid metal located at the tip of the emitter forms a conical protrusion called Taylor Cone, leading to an extraction of ions from the tip.
When such a liquid metal ion source is intended for use in various fields, it is an important requisite that the liquid metal ion source can stably emit an intended ion beam for a long period of time.
Meanwhile, among n-type impurities for silicon semiconductors, the most important elements are arsenic (As) and phosphorus (P) while boron (B) is important with respect to p-type impurities. Phosphorus in the form of a simple substance has a melting point of 44.1.degree. C., and the vapor pressure of P.sub.4 at that temperature is as high as about 24 Pa, which makes it difficult to use phosphorus in the form of a simple substance as a source material for a liquid metal ion source. Similarly, arsenic in the form of a simple substance cannot be used as a ion source, because arsenic in the form of a simple substance has a melting point of 817.degree. C. while its vapor pressure at that temperature is as high as 3.6.times.10.sup.6 Pa. Further, boron in the form of a simple substance is also unsuited as a source material because of its high melting point of about 2400.degree. C.
When an element in a simple substance form which emits an intended ion has a high vapor pressure or a high melting point as mentioned above, the intended element must be converted into an alloy or compound in combination with other elements in order to reduce the above-mentioned difficulties, and the alloy or compound is used as a source material. When the alloy or compound is used as a source material, the emitted ions contain ions of other elements and ions of molecules in combination with other elements besides the intended ion. In such a case, an effectively employed method is one in which a mass spectrometer is provided after the ion source to obtain only the intended ion. In fact, such a method has often been used conventionally. For example, when emission of silicon (Si) ions from a liquid metal ion source is intended, silicon is used as the source material not in the form of a simple substance having a melting point of about 1420.degree. C. but in the form of an alloy thereof with gold (Au), i.e., Au-Si. The melting point of the alloy Au-Si in a eutectic composition form is about 370.degree. C., i.e., much lower than that of silicon. The lowering in melting point advantageously contributes to reduction in electric power consumed during melting as well as reduction in frequency of heat damage to a heater or emitter and prevention of excessive evaporation of the source material.
With respect to extraction of As ions from a liquid metal ion source, the following source materials have been proposed: Sn.sub.68 Pb.sub.24 As.sub.8 reported by Gamo et al. in Jpn. J. Appl. Phys. Vol. 19, No. 10 (Oct., 1980) L. 595 to 598 entitled "B, As and Si Field Ion Sources"; Pd.sub.40 Ni.sub.40 B.sub.10 As.sub.10 reported by Wang et al. in J. Vac. Sci. Technol., Vol. 19, No. 4 Nov./Dec., 1158-1163 (1981) entitled "A mass-separating focused-ion-beam system for maskless ion implantation"; and a Pt-As alloy reported by Shiokawa et al. in J. Vac. Sci. Technol. B, Vol. 1, No. 4, Oct.-Dec. 1, 1117-1120 (1983) entitled "100 keV focused ion beam system with an E.times.B mass filter for maskless ion implantation."
Only one article on extraction of P ions from a liquid metal ion source has been reported by Ishitani et al. in Jpn. J. Appl. Phys., Vol. 23 (1984) L 330-332 entitled "Development of Phosphorus Liquid-Metal Ion Source." In this report, an alloy of copper with phosphorus, i.e., Cu.sub.3 P (P concentration: 25% in terms of the number of atoms) is used as the source material. This report describes that, among emitted ions, P.sup.+ has the highest intensity, and P.sup.2+ has the second highest intensity with respect to phosphorus ions.
Further, with respect to extraction of B ions from a liquid metal ion source, there is an article reported by Ishitani et al. in Nucl. Instrum. & Methods, Vol. 218, 363-367 (1983) entitled "Mass-separated Microbeam System with a Liquid-Metal-Ion-Source." It is not favorable to use metallic materials as materials for an emitter or a heater (reservoir), because boron easily reacts with a metal at a high temperature, leading to a short service life of the ion source. However, in the above-mentioned fifth conventional ion source, an ion source life of 200 hr is attained by using an alloy (melting point: about 1000.degree. C.) represented by the formula Ni.sub.50 B.sub.50 as the source material and using an emitter made of a carbonaceous material called glassy carbon.
The above-mentioned conventional ion sources had the following problems. It is reported that mass analysis of ions emitted by using Sn.sub.68 Pb.sub.24 As.sub.8 as the source material revealed that the amount of the emitted As.sup.+ ions was as small as 0.4% based on the total of the emitted ions, that of As.sup.2+ was 0.1% and As.sup.3+ was 0.1% and that the service life was about 5 hr. As to a Pt-As alloy, it is reported that the life of the ion source was about 10 hr. With respect to the use of CuP.sub.3 as the source material, an apparatus mounting an ion source which uses this source material needs provision of a high-resolution mass spectrometer having a mass resolution of at least 63, because the mass/electric charge ratio, i.e., m/e (m: mass number; e: electric charge number) of P.sup.+ is 31 while that of a divalent Cu ion, i.e., .sup.63 Cu.sup.2+ which is the other one of the elements constituting the source material is 31.5, i.e., the difference in m/e between the two elements is as small as 0.5. Further, it is reported that the service life of the ion source was about 20 hr. The boron ion source proposed by Ishitani et al. which uses an emitter made of a glassy carbon involves a problem that the source materials containing elements capable of emitting intended ions are limited in kind, because metals wettable with a carbonaceous material such as a glassy carbon is limited in kind, e.g., Ni is easily wetted while Pt, Cu, Pd, etc. are difficultly wetted.
As mentioned above, the prior art had various problems such as a short service life of ion sources and a small amount of ionic current with respect to As and P ion sources; and, with respect to B ions, a limited kind of source materials usable for emitting B ions due to a limited kind of metals wettable with a carbonaceous material which has been used for avoiding a reaction between B and the metal. Hence, in the prior art, the liquid metal ion source has not satisfactorily been applied for stably extracting As, P or B ions for a long period of time and for implanting the extracted ions into a Si semiconductor substrate.
In view of the above situations, there has been desired to develop a liquid metal ion source capable of stably emitting As ions, P ions or B ions, or ions of at least one kind of element out of these three kinds of element for a long period of time by making use of a source material which is relatively low in melting point, sufficiently wettable with the emitter, reservoir or heater, small in degree of selective evaporation of As or P and undergoes no significant change in melting point attributable thereto.